Conventionally, in the case of industrially obtaining a regular coffee extract, a method of extracting a prescribed amount of a milled material of roasted coffee beans with hot water by a closed extraction column, a kneader, a decanter or the like is commonly employed. However, according to the method, the extraction operation must be repeated every batch, and the roasted beans whose surface areas have increased by crushing are quickly oxidized, which results in deterioration in flavor and loss of aroma fragrant components. Also, the coffee extract thus obtained by hot water is inevitable from such a defect that even by allowing it to stand for a short time of from about 15 to 30 minutes, the aroma fragrant components which the roasted coffee beans originally have quickly change in quality and get scattered and lost. If it is intended to suppress this compulsorily, a quick cooling step of the coffee extract with a cooling medium as an auxiliary measure becomes necessary; however, a sufficient effect is not obtained.
In this way, according to the hot water extraction that has been carried out on an industrial scale, miscellaneous tastes such astringency or acrid taste components are inevitable, and these miscellaneous tastes strongly appear with an increase of the extraction efficiency. Accordingly, the low-temperature extraction method is proposed, and coffees that are low in astringency and loss of aroma fragrant components, such as water drip coffee (generally called Dutch coffee), are marketed. However, the water drip system or the like, which is the current extraction method and in which the extraction is carried out while dropping water droplets, requires a long time of from 3 to 8 hours for obtaining an extracted liquid having 1% or more of soluble solids. Accordingly, it is extremely difficult to develop commodities that are industrially profitable, and commodities have hardly been put into practical use.
On the other hand, the extraction of green tea as a representative example of teas will be reviewed. In the case of industrial production, in general, the green tea is produced by extracting a prescribed amount of green tea leaves with hot water of 50° C. or higher for 10 minutes or longer using a kneader, a decanter, etc. and removing the used tea leaves by filtration using a flannel filter bag or by using a centrifuge such as a clarifier. In the green tea, the components to be extracted into warm water vary depending upon the extraction temperature. In general, if the extraction temperature is set up at a low temperature, umami components composed mainly of an amino acid such as L-theanine and glutamic acid and various fragrant components are eluted, and if the extraction is set up at a high temperature, astringency components composed mainly of polyphenol, etc. are eluted in addition to the foregoing components. The astringency components are an important factor for forming the flavor of green tea, but excessive elution thereof produces an undesirable flavor.
In the usual case, taking into consideration the economy, the green tea is generally extracted with hot water of 60° C. or higher in industrial production. However, it is also known that the extraction with hot water produces rapid fading or yellowing of pigments such as green tea chlorophyll, which results in a reduction of the quality of color. For preventing this fading, it is considered that the addition of vitamin C or a sodium salt thereof is effective. However, an effect thereof is restrictive, and the addition of an excess of vitamin C or the like produces undesirable results due to the generation of a vitamin odor. Furthermore, according to the treatment at a high temperature for a long time, fragrance and umami that the green tea leaves possess disappear to no small extent, and dietary fibers are hardly extracted.
Also, industrial tea based drinks represented by those in the conventional PET beverage market are made transparent from the viewpoint of appearance by removing the tea grounds that may possibly cause precipitation as far as possible. Furthermore, in order to compensate the browning by heat, it was devised to make the texture of green tea drinks by wrapping with a green film or the like. In this way, so far as the green tea is extracted with hot water, it is inevitable that fading of the extracted liquid of green tea proceeds, whereby a preferable fragrance inherent to the green tea is rapidly lost. Also, conversely, if low-temperature extraction is chosen, since soluble solids are hardly eluted, the extraction requires a long time, which results in a remarkable reduction of the productivity. Accordingly, it is impossible to apply the usual low-temperature extraction method to the industrial production.
Furthermore, in recent years, useful components that a green tea possesses have been studied. In view of the matter that 70% or more of the useful components remain in the tea leaves, a health method such as “tea leaf eating” of directly eating tea leaves has been recently highlighted.
Development of technologies capable of retaining fragrance and umami of teas as far as possible, dispersing useful components of tea leaves represented by dietary fibers in the uniform state in a container for a long period of time and retaining the original green color to considerable extent has been demanded. However, there is no report of complete success yet.